| River barriers change the river connectivity and lead to fragmentation of fish habitats,which result in decline and even extinction of fish populations.In the Lancang River,few studies have studied the impacts of hydropower development on river connectivity,and these studies mainly focused on the impacts of large mainstem dams,but ignored small barriers on tributaries.By conducting a visual interpretation of high-definition remote sensing images,a detailed investigation of river barriers for the whole Lancang catchment was performed,and a complete catchment-scale barrier database was created for the first time.Then,the catchment-scale connectivity was calculated,to assess the current status of river connectivity of the Lancang catchment.In addition,by comparing the visual interpretation barrier database with the existing online barrier database,this study indicates that the existing barrier database significantly underestimates barrier numbers.Furthermore,three representative fish species in the Lancang River,Schizothorax lissolabiatus,Bagarius yarrelli and Percocypris retrodorslis were selected,to analyze the impact of river barriers on their habitat.The main results are as follows:The visual interpretation identified a total of 1052 river barriers from 53 sub-catchments,including 125 large dams,122 small dams,753 weirs and 51 bridge aprons.These river barriers are mainly distributed between1000-2000 meters above sea level.The middle Lancang region has the highest barrier numbers,and the source region has the lowest barrier numbers.Furthermore,barriers are mostly distributed in second order tributaries,within urban and rural areas.The highest barrier density appeared in the middle region and the lowest barrier density appeared in the source region.Across all types of barriers,the middle region has the highest densities of large dam,bridge apron and weir,and the upper region has highest density of small dam.In addition,the source region has the highest DCI_D(diadromous fish)and DCI_P(potadromous fish)values,and the middle region has the lowest DCI values.The results of this study suggest that the middle region is the most fragmented area within the Lancang catchment,and the source region is the least fragmented area so far.On the other hand,the existing barrier database contains only 66 barriers belonging to 30 sub-catchments,which is significantly lower compared with the visual interpretation database.Furthermore,the barrier densities of the visual interpretation database are significantly higher than those of the existing database,and both DCI_P and DCI_D values are significantly lower than those of the existing database.Results of the study showed that using existing online databases to assess connectivity of the Lancang River Basin would underestimate the true fragmentation level of the catchment.By analyzing the impact of river barriers on the habitat of three representative fish species(Schizothorax lissolabiatus,Bagarius yarrelli,and Percocypris retrodorslis),the results showed that the habitat ranges of all three species have decreased over the past few decades.The current distribution range of Schizothorax lissolabiatus has reduced by 23.75%compared with the historical distribution range,and the river barrier density of lost habitat was 92.74%higher than that of existing habitat;both DCI_P and DCI_D values were higher than those in the lost habitat.Compared with the historical distribution,the current distribution range of Bagarius yarrelli has been reduced by47.27%,forming two shrinking areas.The barrier density of larger lost habitat was73.87%higher than the existing habitat,and the barrier density of smaller lost habitat was 8.01%higher than that of the existing habitat.The DCI_P value of the existing habitat was lower than that of the two lost habitats,and the DCI_D index was higher than that of the smaller lost habitat and lower than that of the larger lost habitat.Compared with the historical distribution,the current distribution range of Percocypris retrodorslis has reduced by 72.27%,forming two shrinking areas.The barrier density of the larger lost habitat was 98.60%higher than that of the existing habitat,and the barrier density of the smaller lost habitat was 64.87%higher than that of the existing habitat.The DCI_Pvalue of the existing habitat is higher than that of the larger lost habitat and lower than that of the smaller lost habitat.The DCI_D value of the existing habitat was higher than that of the two lost habitats.The results indicate that constructions of small-and medium-scale barriers,including small dams and weirs are the main reason for the decrease in connectivity and fish habitat loss.In general,this study collected and established the first complete barrier database of the Lancang catchment through visual interpretation method.Then,it revealed the spatial distribution pattern of barriers,as well as the status and spatial differences in connectivity between sub-basins,and assessed the degree of fish habitat loss caused by decreased connectivity.This study provides evidence on the impact of river barriers on river connectivity and fish habitat,and the results can contribute to connectivity restoration and fish habitat protection in the Lancang River. |
